Article pubs.acs.org/JPCC
Charge-Transfer-Induced Photoluminescence Enhancement in Colloidal Silicon Quantum Dots Hiroshi Sugimoto,* Yusuke Hori, Yusuke Imura, and Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *
ABSTRACT: Charge transfer interaction between colloidal silicon (Si) quantum dots (QDs) and adsorbed molecules was investigated by means of photoluminescence (PL) spectroscopy. The molecule employed is tetrathiafulvalene (TTF), which has the highest occupied molecular orbital (HOMO) near that of Si QDs and can act as an electron donor to Si QDs. The energy difference between the HOMOs of TTF and Si QDs was controlled by changing the size of Si QDs. We found that the PL of Si QDs is strongly modified by the adsorption of TTF and that the PL change is reversible, that is, removal of TTF from a colloidal solution recovers the PL intensity. In the smallest Si QDs, where the HOMO level is expected to be 0.11 eV lower than that of TTF, a 2.5fold enhancement of the PL was observed. The PL enhancement suggests that Si QDs having p-type behavior are compensated by electron transfer from TTF, similarly to the case of substitutional phosphorus doping in Si QDs. The observed size dependence of the PL enhancement factors suggests that the charge transfer process is described by classical Marcus theory.
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INTRODUCTION Colloidal semiconductor quantum dots (QDs), which exhibit size-tunable optical and electrical properties, are of interest for optoelectronic applications including photovoltaics1,2 and lightemitting and light-detecting devices.3−5 For the fabrication of high-performance optoelectronic devices, controlling the carrier density is crucial. In bulk semiconductors, this has been achieved by substitutional doping of impurity atoms, which provide extra electrons or holes. The same method can also be applied to semiconductor QDs, and the effect of doping on the conductivity and the luminescence properties have been studied.6−9 In semiconductor QDs, due to the small size and large surface to volume ratio, a more effective route for carrier control has been demonstrated. The method is extraction (injection) of electrons from (to) QDs via chemical bonding with ligands or via the exchange or removal of ligands.10,11 Talapin et al. succeeded in controlling the conduction type and enhancing the channel mobility of a field effect transistor (FET) with a PbSe QDs channel by chemical reaction with hydrazine and ethanedithiol (EDT).12 A drawback of this method is possible formation of surface traps during chemical reaction, which may degrade the stability of QDs. Furthermore, precise control of carrier density is usually not straightforward. Adsorption of molecules with proper energy state structures is also considered to be a promising route for charge carrier injection (extraction) in (from) QDs.13−15 In this method, the charge transfer rate can be controlled by the relative position of the relevant energy states of a QD and a molecule,16 their separation in real space,17 and the number of molecules per QD.18 Shim et al. achieved n-type doping in CdSe QDs by © 2017 American Chemical Society
electron transfer from the highest occupied molecular orbital (HOMO) level of a strong reducing agent to the lowest unoccupied molecular orbital (LUMO) level of a CdSe QD.13 Koh et al. succeeded in controlling the conduction type of a Pb chalcogenide QDs FET by the adsorption of cobaltocene.14 Moreover, carrier generation by charge transfer from photoexcited QDs to adsorbed molecules has been demonstrated by steady-state and time-resolved photoluminescence (PL) spectroscopy.19 The technology of carrier control based on charge transfer has been developed for Cd and Pb chalcogenide QDs. On the other hand, research on Si QDs, which are environmentally more benign and highly compatible with existing semiconductor technology, is quite limited. A rare example is the demonstration of photoinduced charge transfer from colloidal Si QDs to adsorbed molecules by Arrigo et al.20 They succeeded in determining the size dependence of the HOMO and LUMO levels of Si QDs from the degree of PL quenching.20 One of the largest difficulties of charge-transfer-induced carrier control in Si QDs is the long organic molecules necessary to prevent agglomeration by steric barriers.21 The long molecules drastically decrease the charge transfer rate.22 The surface insulating layer also prevents charge transfer between QDs and adsorbed molecules.17,23 To achieve efficient charge transfer, ligand-free all-inorganic Si QDs are preferable. In this work, we employ a Si QD having a surface layer containing large amounts of (several atom %) boron (B) and phosphorus (P).24−26 The layer induces negative Received: April 12, 2017 Published: May 8, 2017 11962
DOI: 10.1021/acs.jpcc.7b03451 J. Phys. Chem. C 2017, 121, 11962−11967
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Figure 1. (a) Photograph of a methanol solution of Si QDs (∼2 mg/mL) grown at 1100 °C. TEM images of Si QDs grown at (b) 1100, (c) 1150, and (d) 1200 °C. (e) High-resolution TEM image of Si QDs grown at 1200 °C. (f) PL spectra of Si QDs with different sizes, that is, grown at different temperatures. (g) HOMO and LUMO levels with respect to the vacuum level of Si QDs with different sizes. The HOMO level of TTF is shown in the right panel.
solutions. The molar ratio of TTF to a Si QD (RTTF/QD) changes over a wide range. Note that TTF does not have PL and absorption bands in the wavelength range studied, and thus, the PL from the mixture solution solely arises from Si QDs (see Supporting Information, Figure S1). Figure 2a shows PL spectra
potential on the surface of Si QDs and makes them electrostatically stable in aqueous solution without organic ligands. The ligand-free surface of Si QDs allows dopant molecules to have access to the surface, and efficient charge transfer interaction is expected. As a dopant molecule, we employ tetrathiafulvalene (TTF), the HOMO level of which is located near that of a Si QD.27,28 We study the charge transfer interaction by monitoring PL properties of Si QDs with different sizes, that is, different HOMO level energies. We show that the effect of TTF on PL properties of Si QDs depends strongly on the size. When the HOMO energy of Si QDs is lower than that of TTF, the PL is strongly enhanced. This suggests that electrons provided by TTF compensate positively charged defects in Si QDs.
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RESULTS AND DISCUSSION Figure 1a shows a photograph of colloidal dispersion of allinorganic Si QDs grown at 1100 °C. The preparation procedure and the fundamental properties are described elsewhere.24,25 The yellowish-brown dispersion is very clear, and light scattering by agglomerates cannot be seen.29 Figure 1b−d shows transmission electron microscope (TEM) (JEOL JEM-2100F) images of Si QDs grown at different temperatures. The average diameters are 3.9, 4.5, and 6.5 nm for Si QDs grown at 1100, 1150, and 1200 °C, respectively.25,30 Figure 1e is a highresolution TEM image of a Si QD grown at 1200 °C, showing a lattice fringe corresponding to the {1 1 1} plane of the Si crystal (lattice spacing: 0.31 nm). Figure 1f shows normalized PL spectra of colloidal Si QDs grown at different temperatures. The PL peak energy can be controlled from 1.0 to 1.4 eV by the growth temperature.25 The size dependence of the HOMO level energy of the Si QDs measured from the vacuum level has been determined by photoemission yield spectroscopy.30 The left panel of Figure 1g shows the HOMO level energies. The energies of the LUMO level obtained by adding the PL peak energies to the HOMO levels are also shown.30 The HOMO level becomes deep as the diameter decreases. In the right panel of Figure 1g, the HOMO level of TTF (−4.97 eV) is shown. The HOMO level is shallower than that of Si QDs 3.9 nm in diameter and is deeper than that for Si QDs 6.5 nm in diameter. The ability to control the relative position of the HOMO levels enables us to study the charge transfer interaction between Si QDs and TTF comprehensively. In this work, we study charge transfer between Si QDs and TTF by monitoring PL spectra of Si QDs in the mixture
Figure 2. PL spectra of Si QDs with different diameters of (a) 3.9, (b) 4.5, and (c) 6.5 nm in mixture solutions with different RTTF/QD values (0.5 (dashed lines) and 5 (dotted lines)). (d) Integral PL intensity of QDs with different diameters as a function of the Si QDs/TTF molar ratio.
of mixture solutions containing Si QDs with an average diameter of 3.9 nm. The spectra are normalized by the peak intensity of Si QDs without TTF (black solid lines). The PL intensity increases significantly by adding TTF. The intensity is enhanced about 2fold when RTTF/QD is 5. Figure 2b shows PL spectra of mixture solutions containing Si QDs 4.5 nm in diameter. The scale of the ordinate is the same as that of Figure 2a. We can thus compare the PL intensity between the two figures. The TTF concentration dependence of the PL spectrum is different from that in Figure 2a. Almost no PL enhancement is observed for RTTF/QD = 0.5, and an enhancement of 1.45-fold is obtained for RTTF/QD = 5. Similar data obtained for Si QDs 6.5 nm in diameter are shown in 11963
DOI: 10.1021/acs.jpcc.7b03451 J. Phys. Chem. C 2017, 121, 11962−11967
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where A is the frequency factor and ΔG* is the free activation energy or the driving force for charge transfer. ΔG* is determined by the free-energy change of the electron transfer step, that is, minus the energy difference between the energy levels of interacting molecules.20,34,64,65 The charge transfer rate increases with increasing ΔG*. As can be seen in the energy state diagram in Figure 1g, electron transfer from TTF to Si QDs is expected to be most efficient for the smallest Si QDs (dave = 3.9 nm). The rate decreases in slightly larger Si QDs (dave = 4.5 nm) because the HOMO levels almost coincide. For the largest Si QDs, the electron transfer is prohibited. These are consistent with the size dependence of the degree of PL change in Figure 2, confirming that the observed PL enhancement is due to compensation of positively charged defects by electron transfer from TTF. The compensation makes a dark Si QD into a “bright” one. In this model, PL decay rates are expected to be not strongly modified by the adsorption of TTF because the PL enhancement is simply due to the decrease of the number of dark Si QDs and the increase of the number of bright ones. In Figure 3, PL
Figure 2c. In this case, no PL increase is observed until RTTF/QD = 5. The observed strong size dependence of the effect of TTF on the PL of Si QDs suggests that a small difference of the relative position of the energy states affects the charge transfer interaction significantly. In Figure 2d, the integral PL intensities are plotted as a function of RTTF/QD for different sizes of Si QDs. The range of RTTF/QD is extended to 5000. In Si QDs 3.9 and 4.5 nm in diameter, the intensity first increases, reaches a maximum at around RTTF/QD = 5, and then decreases, while in Si QDs 6.5 nm in diameter, no increase of the intensity is observed. Before starting discussion on the mechanism of the observed phenomena, it is very important to identify whether the effects are due to the formation of chemical bonding with TTF or the adsorption. We thus removed TTF from the mixture solution by a centrifugal filter (see the Supporting Information for details) and measured the PL spectra. We found that in QDs 4.5 nm in diameter, the intensity recovered to the value of the reference for RTTF/QD = 0.5−500 (Figure S2 in the Supporting Information). The reversible behavior confirms that the observed PL change is due to adsorption or collision interaction with TTF and is not due to the formation of chemical bonding. In smaller QDs, the filtering process did not work well, and thus, we cannot exclude the possibility that other mechanisms are also present in parallel to the adsorption interactions. It should be stressed here that PL quenching by charge transfer has been observed in similar systems31,32 and is explained in general by the introduction of additional nonradiative decay channels in the presence of dopant molecules.31−35 On the other hand, to our best knowledge, chargetransfer-induced PL enhancement has never been reported. In order to discuss the mechanism of the PL enhancement, we first summarize a known mechanism to limit the external PL quantum yield (QY) of Si QDs,36,37 which is usually 20− 30%38,39 if there is no specific surface treatment such as H passivation40 and functionalization with alkyl chains.41,42 A major cause of the low external QY is the existence of a large number of “dark” Si QDs, which do not contribute to PL, in a QD ensemble. A well-known surface defect in Si QDs is a Pb center.43−46 A Pb center captures a photoexcited electron and leaves a hole in a QD.47 Even without photoexcitation, a positively charged Pb center located close to the valence band maximum draws an electron and generates a hole.48−51 In a Si QD with an excess hole, a photoexcited electron and hole pair recombines via an Auger process, which is more than 3 orders of magnitude faster than the radiative recombination.52 As a result, the internal QY of a Si QD with a Pb center becomes practically zero. The same scenario may be applied to the present allinorganic Si QDs because the surface structure estimated from infrared absorption spectroscopy does not change so much from that of conventional Si QDs.53 In Si QDs embedded in solid matrixes and Si QD powder, a Pb center is effectively deactivated by substitutional P doping, which results in the enhancement of the PL external QY.54−59 The most accepted scenario of the PL enhancement is that an electron supplied by substitutional P compensates a Pb center and makes a p-type Si QD an intrinsic one.54,58,60−63 It is very plausible that the PL enhancement in Figure 2 can be explained by the same model; electron transfer from an adsorbed TTF molecule to a Si QD compensates a hole in a dark Si QD, resulting in the PL enhancement. This model explains the mechanism that the effect of TTF on PL depends strongly on the size of Si QDs. In classical Marcus theory, the charge transfer rate (kc) is described as kc = A exp(−ΔG*/RT),
Figure 3. Decay rates of 3.9 (red), 4.5 (green), and 6.5 (blue) nm Si QDs as a function of the Si QDs/TTF molar ratio. The broken lines show the decay rates of references.
decay rates obtained by fitting the decay curves by a stretched exponential function are plotted as a function of the amount of TTF (see the Supporting Information for the decay curves and the procedure to obtain decay rates). In all three sizes, the decay rate is almost constant up to RTTF/QD = 50, despite the fact that the PL enhancement factor depends strongly on the size. This supports the validity of our model. Another possible explanation of the size dependence of the degree of PL enhancement is that a fraction of dark Si QDs is different between samples with different size QDs and that larger enhancement can be achieved in the samples with a larger fraction of dark Si QDs. However, in the present samples, the PL QY is larger in smaller QDs, that is, 4.5, 1.7, and 0.2% for 3.9, 4.5, and 6.5 nm in diameter QDs, respectively.25 Therefore, this model is not appropriate to explain the observed size dependence. In the TTF concentration range of RTTF/QD ≥ 500, the PL intensity starts to decrease in all three sizes. In this range, the PL quenching is accompanied by the increase of the decay rates. This suggests that in very large RTTF/QD, a nonradiative process is introduced in Si QDs. Considering the very large RTTF/QD, the quenching may be due to chemical reaction between TTF and Si QDs and the formation of chemical bonds. In fact, in RTTF/QD = 5000, PL change by TTF adsorption is not perfectly reversible (Figure S2). There have been several literature reports of PL quenching of Si QDs due to chemical reaction with sulfur11964
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The Journal of Physical Chemistry C containing molecules.66,67 We consider that a similar phenomenon occurs in the case of TTF (C6H4S4) when RTTF/QD is very large, although the number of chemical bonds is too small to be detected by FTIR. The rate and efficiency of electron transfer from an adsorbed TTF molecule to a Si QD should decrease exponentially with the distance because it is a tunneling process.17,22 In fact, in organic-ligand-capped QDs, the charge transfer rate is reported to depend strongly on the length of the ligand.22 In the present Si QDs, the surface is predominantly H-terminated just after preparation, and during storage in methanol, the surface is slowly oxidized. This can be seen in the number ratio of Si−O and Si−H bonds estimated from the intensities of IR absorption bands of Si−O−Si (1080 cm−1) and Si−H (2100−2200 cm−1) stretching modes by taking into account the oscillator strengths (Figure 4a) (see the Supporting Information for details).68,69
PL enhancement becomes smaller with increasing storage term and no enhancements are observed after storage for 90 days. Therefore, the electron transfer from TTF to Si QDs is very sensitive to the surface potential barrier.
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CONCLUSIONS
We have succeeded in demonstrating charge transfer interaction between Si QDs and TTF. Electron transfer from TTF compensates positively charged defects in Si QDs and enhances the PL. We observed clear size dependence on the PL enhancement factors by TTF adsorption. The size dependence could be well explained by the relative relation between the HOMO levels of a TTF molecule and a Si QD. We also showed that the electron transfer is very sensitive to the surface structure of a Si QD and a slight increase of the potential barrier smears the electron transfer.
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EXPERIMENTAL DETAILS Preparation of Colloidal Si QDs. Colloidal Si QDs were fabricated by a cosputtering method as described in our previous paper.24,25 Si, SiO2, B2O3, and P2O5 were simultaneously sputtered, and a mixture film was deposited on a stainless steel substrate. The film was peeled from the plate and annealed in a N2 gas atmosphere at 1100, 1150, and 1200 °C for 30 min to grow Si QDs in a BPSG matrix. Si QDs were extracted from a matrix by a hydrofluoric acid (HF) solution and dispersed in methanol. Preparation of Si QD−TTF Solutions. First, a methanol solution of Si QDs was divided into five portions to prepare Si QD solutions with exactly the same QD concentration. Methanol solutions of TTF (Sigma-Aldrich) with different concentrations were added to the Si QD solutions. The TTF concentrations were chosen so that the molar ratio of TTF to Si QD (RTTF/QD) became 0.5, 5, 50, 500, and 5000. The total amount of solutions was fixed. Note that in the RTTF/QD range, Si QD dispersed solutions are very stable and no agglomerates are formed by mixing. In this work, the effect of TTF concentration on the optical properties of Si QDs was studied for three series of samples containing different sizes of Si QDs. The QD concentrations are 4.9 × 1015, 4.6 × 1015, and 5.3 × 1015/mL for samples containing 3.9, 4.5, and 6.5 nm QDs. Optical Measurement. PL spectra were collected by a spectrofluorometer (Fluorolog- 3, HORIBA Jovin Yvon) equipped with a photomultiplier (500−850 nm) and an InGaAs photodiode (800−1500 nm). The excitation source was a monochromatized Xe lamp (405 nm). PL decay curves at different emission energies were measured using a NIR photomultiplier (R5509-72, Hamamatsu Photonics) and a multichannel scalar (SR430, Stanford Research). The excitation source was modulated 405 nm light (CUBE 1073840, COHERENT). All of the measurements were carried out at room temperature.
Figure 4. (a) Number ratio of Si−O−Si and Si−H bonds obtained from IR absorption spectra as a function of the storage term in methanol. XPS spectra of co-doped Si QDs (b) 1 and (c) 90 days after preparation. (d) Integral PL intensities of colloidal Si QDs as a function of TTF concentration. The storage terms are 20 (black), 30 (orange), and 90 days (red). Lines are guides for the eyes.
Similarly, in Si 2p X-ray photoelectron spectra, the change of the surface structure during storage in methanol can be seen. In the spectrum 1 day after preparation (Figure 4b), the signal is dominated by Si0 (99.6 eV) from a Si QD core and Si3+ (102.5 eV) from the surface suboxides, while after storage in methanol for 90 days (Figure 4c), the oxide signal shifts to 103.6 eV (Si4+), that is, stoichiometric SiO2, and the intensity of the Si0 peak decreases.70,71 In Figure 4d, the effects of TTF adsorption on the PL intensity of Si QDs (dave = 3.9 nm) are compared between three samples with different storage terms. We can clearly see that the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03451. Photoluminescence and absorbance spectra of Si QDs and TTF, reversible behavior of PL intensity after removal of TTF, representative PL decay curves, and detailed analyses of IR absorption spectra (PDF) 11965
DOI: 10.1021/acs.jpcc.7b03451 J. Phys. Chem. C 2017, 121, 11962−11967
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.S.). *E-mail:
[email protected] (M.F.). ORCID
Hiroshi Sugimoto: 0000-0002-1520-0940 Minoru Fujii: 0000-0003-4869-7399 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by the 2015 JST Visegrad Group (V4)−Japan Joint Research Project on Advanced Materials and JSPS KAKENHI Grant Number 16H03828.
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REFERENCES
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DOI: 10.1021/acs.jpcc.7b03451 J. Phys. Chem. C 2017, 121, 11962−11967
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DOI: 10.1021/acs.jpcc.7b03451 J. Phys. Chem. C 2017, 121, 11962−11967
